1. Field of the Invention
The present invention relates generally to the diversity reception of a transmitted signal using an array of antennas, and more specifically to the performance assessment of an array of antennas in a switched space diversity receiving system.
3. Description of the Related Art
Diversity combining is used in a space-diversity technique for combining signals received via independent propagation paths in such a way that a high signal to noise ratio is achieved. This technique is suitable for applications where a deep fade occurs in the propagation paths. However, if the fading pattern of a propagation path is obscure such as the one that occurs frequently during a cellular mobile communication, diversity combining technique is not suitable. In such instances, a switched space diversity technique is employed for selecting the highest of the received signals.
A fault detection technique for an array of antennas is disclosed in Japanese Laid-Open Patent Specification Hei-9-83418. According to this prior art, signals from an array of antennas are monitored to determine their strength values and the highest value is selected as a reference. Differences between the strength value of the antennas and the reference are determined respectively for the antennas and then compared with a threshold. Counters are associated respectively with the antennas. If one of the difference values exceeds the threshold, the counter associated with that difference value is incremented. This counter is reset to zero whenever that difference value is lower than the threshold. If the counter has been incremented to a value exceeding some critical value, an alarm indication is given that the antenna associated with this counter is not functioning properly.
However, the detection of performance degradation of a whole array of antennas, rather than with individual antennas, is also important. Although detecting electromagnetic reflections from the antennas or subjecting them to switching between a d.c. short-circuit and an open- circuit condition would be one solution, the hardware installation cost cannot economically be justified.